I use fox farm nutrients, and i follow what they say. I change the res, every week or every other week, according to what it says. I also follow their flush timeline, unless my plants show signs they need it earlier. If you want to play it totally safe, do every week. If you do every other week, make sure you top it off with a nutrient/water mix. Some people only change their res once, between veg and flowering. They just always top it off, and get the ppms back to what it should be.